As protein fibers, fibroin fibers that are regenerated silk fibers, artificial spider silk fibers, and the like are known. Some solution-dyed fibers of these protein fibers have already been proposed. For example, Patent Document 1 proposes a method for producing a regenerated silk fiber, including; adding silk fibroin and hematin into a hexafluoroisopropanol (HFIP) solvent; extruding the solution into a methanol coagulation liquid for spinning; and performing cold drawing. As the artificial spider silk fibers, for example, Patent Document 2 describes addition of Sudan red or Nile red (both of them are pigments) into a spinning solution in Example 6, and addition of Green Fluorescent Protein (GFP) into a spinning solution in Example 7.
However, there are problems with the dispersibility of such pigments into a spinning solution and the cost of GFP. Poor dispersibility or solubility of pigments into a spinning solution will result not only in yarn breakage in a spinning process but also difficulty in obtaining solution-dyed fibers with uniform composition. Additionally, bright colors are less likely to be exhibited.